In venetian blinds, particularly when they are used externally of the window, it is frequently desirable that the same be locked in the lowered position in order to prevent rattling due to a strong wind and also for security purposes against possible unauthorized entry. It is also desirable that such a locking device be simple in construction and assembly for purposes of economy and equally as simple in operation. Additionally, such a locking device should preferably not require separate keys or the like which tend to become lost or misplaced.